EFFECT OF RESISTANCE TRAINING ON MUSCLE USE DURING EXERCISE

This study examined the effect of resistance training on exercise-indu ced contrast shift in magnetic resonance (MR) images. It was hypothesi zed that a given load could be lifted after training with less muscle showing contrast shift, thereby suggesting less muscle was used to per form the exercise. Nine males trained the left quadriceps femoris (QF) muscle 2 days/wk for 9 wk using 3-6 sets of 12 knee extensions each d ay. The right QF served as a ''control.'' Exercise-induced contrast sh ifts in MR images evoked by each of three bouts of exercise (5 sets of 10 knee extensions with a load equal to 50, 75, and 100% of the maxim um pretraining load that could be lifted for 5 sets of 10 repetitions) were quantified pre- and posttraining. MR image contrast shift was qu antified by determining QF cross-sectional area (CSA) showing increase d spin-spin relaxation time. One repetition maximum increased 14% in t he left trained QF and 7% in the right untrained QF. Left QF CSA incre ased 5%, with no change in right QF CSA. Left QF CSA showing contrast shift was less after each bout of the exercise test posttraining. This was also true, to a lesser extent, for the right QF at the higher two loads. The results suggest that short-term resistance training reduce s MR image contrast shift evoked by a given effort, thereby reflecting the use of less muscle to lift the load. Because this response was ev ident in both trained and contralateral untrained muscle, neural facto rs are suggested to be responsible. The consequence of this adaptation could be to increase ''stress'' per unit area of active muscle during the course of training and thereby evoke hypertrophy.